The present invention relates generally to data transmission in communication systems and, more specifically, to methods and systems for channel state feedback in networks and devices implementing carrier aggregation.
As used herein, the terms “user equipment” and “UE” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other User Agents (“UA”) that have telecommunications capabilities. In some embodiments, a UE may refer to a mobile, wireless device. The term “UE” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station or other network node transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems and equipment result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” will refer to any component or network node, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with access to other components in a telecommunications system.
In mobile communication systems such as E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel to be used for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UEs through a control channel.
To facilitate communications, a plurality of different communication channels are established between a base station and a UE including, among other channels, a Physical Downlink Control Channel (PDCCH). As the label implies, the PDCCH is a channel that allows the base station to control a UE during downlink data communications. To this end, the PDCCH is used to transmit scheduling assignment or control data packets referred to as Downlink Control Information (DCI) packets to a UE to indicate scheduling to be used by the UE to receive downlink communication traffic packets on a Physical Downlink Shared Channel (PDSCH) or transmit uplink communication traffic packets on a Physical Uplink Shared Channel (PUSCH) or specific instructions to the UE (e.g., power control commands, an order to perform a random access procedure, or a semi-persistent scheduling activation or deactivation). A separate DCI packet may be transmitted by the base station to a UE for each traffic packet/sub-frame transmission.
It is generally desirable to provide high data rate coverage using signals that have a high Signal to Interference Plus Noise ratio (SINR) for UEs serviced by a base station. Typically, only those UEs that are physically close to a base station can operate with a very high data rate. Also, to provide high data rate coverage over a large geographical area at a satisfactory SINR, a large number of base stations are generally required. As the cost of implementing such a system can be prohibitive, research is being conducted on alternative techniques to provide wide area, high data rate service.
In some cases, carrier aggregation can be used to support wider transmission bandwidths and increase the potential peak data rate for communications between a UE, base station and/or other network components. In carrier aggregation, multiple component carriers are aggregated and may be allocated in a sub-frame to a UE as shown in FIG. 1. FIG. 1 shows carrier aggregation in a communications network where each component carrier has a bandwidth of 20 MHz and the total system bandwidth is 100 MHz. As illustrated, the available bandwidth 100 is split into a plurality of carriers 102. In this configuration, a UE may receive or transmit on multiple component carriers (up to a total of five carriers 102 in the example shown in FIG. 1), depending on the UE's capabilities. In some cases, depending on the network deployment, carrier aggregation may occur with carriers 102 located in the same band and/or carriers 102 located in different bands. For example, one carrier 102 may be located at 2 GHz and a second aggregated carrier 102 may be located at 800 MHz.
In network communications, information describing the state of one or more of the carriers or communication channels established between a UE and a base station can be used to assist a base station in efficiently allocating the most effective resources to a UE. Generally, this channel state information (CSI) includes measured CSI at a UE and can be communicated to the base station within uplink control information (UCI). In some cases, in addition to the CSI, UCI may also contain Hybrid Automatic Repeat reQuest (HARQ) acknowledgment/negative acknowledgement (ACK/NACK) information in response to PDSCH transmissions on the downlink. HARQ ACK/NACK transmissions are used to signal successful receipt of data transmissions and to request retransmissions of data that was not received successfully. Depending upon the system implementation, the CSI may include combinations of one or more of the following as channel quality information: Channel Quality Indicator (CQI), Rank Indication (RI), and/or Precoding Matrix Indicator (PMI). For LTE-A (Rel-10), depending upon the system implementation, there may be more channel quality information types in addition to the formats listed above.
The CSI provides information about the observed channel quality on a downlink carrier observed by the UE. The base station then uses the CSI to assist with downlink scheduling and other applications. For example, the CQI may assist the base station with selecting an appropriate modulation and coding scheme (MCS). The RI provides an indication as to whether the UE can support one or multiple spatial multiplexing layers, and the PMI provides information about the preferred multi-antenna precoding for downlink transmissions.
Depending upon the uplink transmission resources available at a particular point in time, the UE may transmit the CSI information within UCI either on a Physical Uplink Control CHannel (PUCCH) resource or multiplexed into a PUSCH (Physical Uplink Shared CHannel) allocation.
A PUCCH format 2/2a/2b (see, for example, TS 36.211, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)”. http://www.3gpp.org/ftp/Specs/html-info/36211.htm) may be used for CSI transmission in Rel-8 if no PUSCH allocation is scheduled. This PUCCH format can carry 20 coded bits corresponding to a maximum information bit payload of about 11 CSI bits, and the CSI payload may be block-encoded as described in section 5.2.3 of TS 36.212, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”. http://www.3gpp.org/ftp/Specs/html-info/36212.htm.
If a PUSCH allocation is available, the CSI information may be first encoded and then multiplexed with an uplink shared channel (UL-SCH) transport block as described in sections 5.2.2.6 and 5.2.2.7 of TS 36.212, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”. http://www.3gpp.org/ftp/Specs/html-info/36212.htm, respectively. If no UL-SCH transport block is present, then the CSI information may be encoded to fill the PUSCH allocation as described in section 5.2.4 of TS 36.212, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”. http://www.3gpp.org/ftp/Specs/html-info/36212.htm.
For reference, section 8.6.2 of TS 36.213, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”. http://www.3gpp.org/ftp/Specs/html-info/36213.htm describes how a base station can provide a PUSCH grant and signal to the UE that the PUSCH allocation is to be used only for control information feedback (i.e. no UL-SCH transport block is to be included).
While a UE is communicating with a base station, uplink carriers and downlink carriers may be activated or deactivated depending upon the resource allocations made by the base station to the UE. Generally, carrier activation and deactivation can be accomplished through either explicit or implicit activation or deactivation. Explicit activation of configured downlink carriers may be performed using media access control (MAC) signaling through, for example, a MAC control element (CE). Similarly, explicit deactivation of configured downlink carriers can be performed using MAC signaling (e.g., using a MAC control element). Implicit deactivation of configured downlink carriers may be performed using a timer associated with each activated downlink carrier. In that case, the timer for a particular carrier is reset whenever any activity (i.e. a transmission or a retransmission) occurred on that carrier. If the timer expired through a lack of activity, the corresponding downlink carrier may then be implicitly deactivated by the UE.
In a multi-carrier network implementation providing the functionality described above (e.g., allowing for explicit or implicit deactivation of carriers), there are several important considerations. A UE may be assigned certain uplink resources for reporting CSI information about the currently activated downlink carriers. In that case, it is important to ensure that those resources are efficiently used for transmitting CSI from a potentially variable number of downlink carriers. Furthermore, it is important that a UE be capable of indicating to a base station which (and possibly how much) CSI information (i.e. for which downlink carriers) is included in a particular control feedback transmission.